Source: Penn State Extension
By Douglas Beegle, Agronomist
At this point many fields look pretty good but we are seeing some indication of nitrogen (N) deficiency around Pennsylvania due to the higher than normal rain earlier this summer.
So what happened with your N? A quick assessment you can do as the crop reaches silage harvest time and maturity is to visually assess the crop by the number of green leaves below the ear. Generally if you have at least 4 green leaves at and below the ear at silage harvest time, the crop had adequate N. This assessment is based on the assumption that the yellow or brown leaves at the bottom of the plant are due to N deficiency.
Other things such as disease and dry conditions will also cause the lower leaves to senesce. Be careful not to confuse this with N deficiency. Nitrogen deficiency is characterized by a distinct “V” shaped pattern starting at the tip of the leaf and going back the mid-rib on the lower leaves on the plant. Drought senescence and diseases usually do not follow this very distinct pattern.
The Late Season Cornstalk Nitrate Test (LSSNT) has been demonstrated to be a reliable end-of-season indicator of crop N status. It provides a good assessment of whether the crop had the right amount of N, too much N, or whether it ran out of gas. This information combined with records of N management can be very useful for making and fine-tuning future N and manure management decisions. See Agronomy Factsheet #70 Late Season Cornstalk Nitrate Test for all of the details. Here is a video on collecting samples for the LSSNT.
For the test results to be valid, the sampling instructions must be followed carefully. Samples for this test should be taken between ¼ milk line (silage harvest stage) and up to 3 weeks after black-layer. An 8-inch-long section of corn stalk starting 6 inches above the ground is collected from at least 10 representative plants in a field. After the sample is collected the stalk segments should be cut into shorter pieces to promote drying and then sent to the lab for analysis. Most labs that do business in Pennsylvania, including the Penn State Ag Analytical Services Lab, will run the LSSNT.
Special Note for Sampling Following Harvest
Research has shown that samples could be taken up to 24 hours following silage harvest. For this to work the corn must be chopped at least 14 inches high where the sample is to be taken so that you can still get the correct sample, i.e., 8-inch-long sections of corn stalk starting 6 inches above the ground. Some farmers will raise the chopper head occasionally to leave some taller stubble to facilitate sampling later. Don’t delay any longer, and in general, be careful if there is any stalk deterioration.
If the results of the test fall between 700 and 2,000 ppm Nitrate-N, this indicates that the N management was optimum. Below this range the crop likely ran out of N and did not achieve full yield potential. Note that the test drops off dramatically when the crop becomes N deficient.
Consequently, the test is not very sensitive to how deficient the crop is. Results above this range indicate that the crop had more than enough N, which could represent an economic loss from purchasing unnecessary fertilizer N or wasted manure N and it could result in increased potential for loss of N to the environment. Compare the results with management records to determine the possible reasons for the outcome and use this to make adjustments in future management.
The key thing will be to look at what the test is telling you in the context of the crop management. Specifically, timing, method and form of N applied are important factors to evaluate in relation to the LSSNT. If N amendments were used, the LSSNT could help indicate how well these worked under the conditions this year. This would be especially helpful if there were check plots for comparison.
The best value from this test is to evaluate N management over time, and in the context of the conditions each year. Good records of management and growing conditions can be very helpful.